1. Field of the Invention
The present invention relates to a heat shield comprising a light-reflective layer formed of a fixed cholesteric liquid-crystal phase, and to laminated glass having it. The heat shield and the laminated glass of the invention are useful for heat shield for windowpanes for automobiles.
2. Background Art
With the recent increase in interest in environment and energy-related issues, the needs for energy-saving industrial products are increasing; and as one of them, glass and film are desired that are effective for heat shield for windowpanes for houses, automobiles, etc., or that is, effective for reducing heat load due to sunlight. For reducing heat load due to sunlight, it is necessary to prevent transmission of sunlight rays falling within any of the visible range or the infrared range of the sunlight spectrum. In particular, windowpanes for automobiles are required to have high transmittance of visible light from the safety viewpoint, and are additionally required to have a high level of heat shieldability. In the State of California in US, there is a move to control Tts (total solar energy transmitted through glazing).
Laminated glass coated with a special metallic film capable of blocking out thermal radiations, which is referred to as Low-E pair glass, is often used as eco-glass having high heat-insulating/heat-shielding capability. The special metallic film may be formed by lamination of plural layers, for example, according to a vacuum-deposition method disclosed in JP-A 6-263486. The special metallic film formed through vacuum deposition is extremely excellent in reflectivity, but the vacuum process is nonproductive and its production cost is high. In addition, when the metallic film is used, it also blocks electromagnetic waves; and therefore in use in mobile telephones and the like, the metallic film may causes radio disturbance; or when used in automobiles, there may occur a problem in that ETC (electronic toll collection) could not be used. Not only for evading the problem of radio disturbance but also from the safety viewpoint, windowpanes for automobiles are required to have high transmittance of visible light.
For solving the problem of radio disturbance, for example, JP-A 2002-131531 discloses a method of using a layer containing metallic fine particles. The metallic fine particles-containing film is excellent in visible light transmittance but has a low reflectivity to light falling within a wavelength range of from 700 to 1200 nm that significantly participates in heat shielding, and therefore has a problem in that its heat-shielding capability could not be enhanced.
For preventing reduction in visible light transmittance and for lowering insolation transmittance falling within a range of from 700 to 800 nm, for example, JP-A 6-194517 proposes a method of coating glass with a layer containing an IR-absorbing dye. Use of an IR-absorbing dye may lower insolation transmittance but is problematic in that the film surface temperature rises through insolation absorption and the heat-shielding capability of the film lowers through re-release of the heat.
For satisfying both high visible light transmittance and high reflectivity in a wavelength range of from 700 to 1200 nm, for example, JP-T 2002-509279 discloses a method of using a birefringent multilayer dielectric film. However, the method is problematic in that, when the front reflection zone of 1000 nm is controlled over the near-infrared range, then the reflection at around 400 nm may rather increase to cause a problem of color shift, and therefore the wavelength control is difficult.
There is known a method of using a cholesteric liquid-crystal layer. For example, as disclosed in Japanese Patent 4109914, one cholesteric liquid-crystal layer may be formed on both surfaces of a λ/2 plate for securing efficient and selective reflection of a light circularly polarized in one direction and falling within a range of from 700 to 1200 nm.
JP-T 2009-514022 discloses an IR-reflective article having a cholesteric liquid-crystal layer. Many trials of using a laminate of plural cholesteric layers in a liquid-crystal display device have been made, and concretely, there are known many trials of efficiently reflecting a light falling within a visible light range. For example, Japanese Patent 3500127 discloses examples of a lamination of a lot of cholesteric layers.
In laminating plural cholesteric layers, there is employed a method of superposing wet coating films of a cholesteric liquid-crystal material one after another through drying, thermal alignment and UV curing thereof. For curing the cholesteric liquid-crystal layer, for example, as exemplified in Japanese patent No. 4008358, there may be generally employed a method of irradiating a polymerizing liquid crystal with UV rays for curing thereof, and for example, there is disclosed a method of forming a wide-area cholesteric liquid-crystal film by controlling the radiation intensity within a predetermined range. Japanese Patent 3745221 discloses a method of forming a continuous wavelength-range polarizing element by laminating plural layers of liquid-crystal molecules to give a multilayer film with so controlling the molecules in each layer as to have the same rotation direction.